The present invention relates to a power source control apparatus for use in electronic equipment of the type using a microprocessor and powered by a main power source in the form of a battery. More particularly, the present invention is concerned with a power source control apparatus for controlling a boosting circuit which boosts a battery voltage for producing an operating voltage of electronic equipment.
Electronic equipment using a microprocessor and including a main power source in the form of a battery is extensively used today. A power source control apparatus for this type of electronic equipment is constituted by a power source or battery, a power switch and a boosting circuit which are connected in parallel to the battery, and a voltage detecting circuit connected to the boosting circuit. In response to an operator's power ON or power OFF operation, the power switch delivers a power ON or a power OFF signal to the boosting circuit while feeding it to external electronic equipment via a power ON/OFF signal output terminal. The boosting circuit is implemented by a DC/DC converter, for example. In response to the power ON signal, the boosting circuit boosts the output voltage (e.g. +1.5 volts) of the battery to +5 volts, for example, while the power ON signal is present. The output voltage of the boosting circuit is fed to the external electronic equipment via a power output terminal for operating it and to the voltage detecting circuit.
The voltage detecting circuit is responsive to a positive-going edge and a negative-going edge of the operating voltage (+5 volts) and delivers a reset signal representative of those edges to the electronic equipment via a reset signal output terminal. Consequently, upon the turn-on of the power switch, the electronic equipment is supplied with the operating voltage (+5 volt) to start on a predetermined operation thereof. When the power is turned on, the external equipment executes predetermined initialization processing in response to a reset signal; when the power source is turned off, it executes various kinds of processing such as memory back-up processing and switching various peripheral circuits associated with the microprocessor to a power saving mode operation, during the interval between the appearance of a power OFF signal and that of a reset signal.
In the prior art power source control apparatus described above, the boosting circuit stops its operation immediately after the turn-off of the power switch and, hence, the operation voltage of the electronic equipment is lowered relatively rapidly after the turn-off of the power switch until it disappears. Since the voltage detecting circuit generates a reset signal as soon as the voltage level being lowered after the turn-off of the power switch crosses a predetermined level, the equipment has to complete the various kinds of processing previously stated within the interval between the reception of the power OFF signal and the reception of the reset signal. Such an interval is so short that the contents or the range which can be processed is limited. An approach heretofore adopted with prior art electronic equipment of the type using a microprocessor for eliminating this problem is to execute particular kinds of processing which have priority over the others in the event of the turn-off of the power source by way of example. However, such a scheme is not easy to practice.
In the initial stage of operation of the boosting circuit, its output voltage is unstable. Hence, when the power switch of equipment of the above-described type is turned on and off repetitively, if not ordinarily, within a short period of time, accurate reset signals cannot be fed to the microprocessor and therefore bring the latter out of control. Further, such electronic equipment is sometimes provided with a back-up RAM so that the RAM may be powered by the battery upon the turn-off of the power switch so as to hold data therein. In this case, there is a fear that the data held in the RAM are disturbed or altered by external noise. Electronic equipment has heretofore been provided with no implementation for eliminating such an occurrence.